I recently installed the WP Super Cache plugin on most of my WordPress blogs. This plugin in makes blogs much speedier to view by caching pages as static HTML files. The first time someone visits a page, it is served up as PHP like normal and the page is saved as an HTML file, and the next time someone views the page, no PHP is run, the page is served up from a cache, which is quicker for the visitor and saves a lot of processor time on the server.
The one downside of the plugin is that you have to remember to disable it when making any changes to your site, otherwise you don’t see the changes. I keep forgetting to do that and find myself cursing when my changes don’t work.